1. Field of the Invention
The present invention generally relates to a magnetic disk medium and a method of producing same.
2. Description of the Related Art
Magnetic disk drives, as used for mass data storage in computers and electronic systems today comprise either rigid ("hard") disk drives or flexible ("floppy") drives. Both types of drives incorporate low cost data storage capacity with rapid recovery of stored data. This rapid availability of stored data is a function of the rotational speed of the disk relative to the read/write transducer as well as the proximity of the transducer to the magnetic medium. In practice, a read/write transducer is mounted in a head assembly that accurately follows the surface of a disk at flying heights of less than 1 micron. In particular, the head suspension assemblies are designed to prevent contact between the read/write head and the magnetic recording medium during operation; such contact, called head "crash", can destroy a read/write head and the magnetic medium in a short time due to the friction that results from the high rotational speed of the disk relative to the head. Current technology for lubrication and protective layers on the disk is basically intended to compensate only for transient friction events during stop/start cycles.
In general, control of the texture characteristics of the disk surface is required to reduce the substantial attractive forces that are generated between the read/write head and the stationary disk surface. Smoother disk surface textures result in higher attractive forces that prevent proper head liftoff and flying characteristics when disk rotation is commenced. Current disk manufacturing techniques must also assure that the disk surface roughness does not exceed certain upper-bound values; if excessive surface roughness results from the texturing process, undesirable increases in flying height (H.sub.0) also limit the density with which data can be stored on the disk. The central issue in current disk texturing processes is the reliability and consistency with which the desired surface roughness is obtained. The disk surface texture is typically characterized in terms of an arithmetic average roughness value (R.sub.a). Current disk texturing processes generally produce R.sub.a values in the range of 10-200 nm; the most modern disk drives achieve head flying heights of 0.2-0.3 microns with R.sub.a values of approximately 10-50 nm. These texturing processes utilize special abrasives for producing circumferential patterns of scratches on the surface of metallic (predominantly aluminum) disk substrates. U.S. Pat. Nos. 4,996,622, 4,939,614 and 4,931,338 describe variations of this general process. Several of these patents propose different textures for separate areas of the disk optimized for stop/start operations and for read/write operations. These patents document the difficulty of obtaining low flying heights (H.sub.0 less than 0.3 microns) while simultaneously achieving acceptably low head/disk attractive forces with current disk texturing processes.
Other texturing processes combine abrasive texturing processes with chemical processes. U.S. Pat. No. 4,985,306 describes a recording disk produced by subjecting a base plate containing S.sub.i O.sub.2 --Li.sub.2 O--A1.sub.2 O.sub.3 series crystallized glass to crystallizing treatment, polishing the surface of the base plate to attain a surface roughness of 15 .ANG. to 50 .ANG. to evenly distribute, regularly and two-dimensionally, very fine and uniform crystal grains in the amorphous layer. The base plate is then etched with an etchant having different degrees of dissolution with respect to the crystal grains and the amorphous layer to form uniform and regular convexities and concavities on the surface of the base plate. A magnetic film and a protective layer are applied over the base plate. Because the system described in this patent relies on an abrasive texturing process for distributing crystal grains, there is an inevitable randomness to the ultimate distribution of concavities and convexities.
The trend toward smaller diameter disks has also presented difficulties for prior-art manufacturing techniques. It has become progressively more difficult to achieve the required consistency in R.sub.a values and in disk flatness with decreasing disk diameter using conventional methods. Disk flatness variations cause axial runout of the read/write head during disk rotation. In current disk drives it is desirable to maintain this axial runout value at less than 1-2 microns. Conventional abrasive texturing techniques applied to current metallic disk substrates are becoming less viable as disk diameters are progressing downwards.